Back to the original poster.....
I don't know how much power you want to make. If you want to make from
500 to 1600 watts of Sine wave AC, forego all "build your own from scratch" ideas and look up "redi-Line" on Ebay.
These are "motor generators", designed to take DC voltage from a car battery 12-36 volts), turn a motor, which in the end drives a generator that puts out Pure sine wave (Clean) power.
These are the precurser to newer electronic "inverters". Though not as "efficient" as newer electronic units (of which you likely do not care
- running Steam and all), this should work Great! These can be had on the used market for far less than the Electronic types (you couldn't use the electronic types for what you are doing anyhow !)
Your mission would be to simply dissassemble the device (you pick the power level you want - I would advice the 1600 watt cause you will likely have the power to turn it) - and expose the motor shaft of the unit, and drive the generator via your Steam engine (3600 rpm is the recommended at full load).
Cool Eh ? Run the microwave via steam power !
I want to see pictures when you are done !